Method and apparatus for production of uniformly sized nanoparticles

ABSTRACT

An apparatus and process for creating uniformly sized, spherical nanoparticles from a solid target. The solid target surface is ablated to create an ejecta event containing nanoparticles moving away from the surface. Ablation may be caused by laser or electrostatic discharge. At least one electromagnetic field is placed in front of the solid target surface being ablated. The electromagnetic field manipulates at least a portion of the nanoparticles as they move away from the target surface through the electromagnetic field to increase size and spherical shape uniformity of the nanoparticles. The manipulated nanoparticles are collected.

TECHNICAL FIELD OF THE INVENTION

The present invention relates to novel processes and apparatus for thepreparation of uniformly sized nanoparticles of various shapes. Becauseof the ability to produce uniformly sized nanoparticles, these particlesexhibit unique characteristics in liquid solutions due to their abilityto remain suspended in solution without the need for surfactants orother stabilizing agents.

BACKGROUND OF THE INVENTION

Over the past two decades substantial effort has been directed to thecreation and study of what are commonly called nanoparticles, despitethe fact that numerous definitions of what qualify as so-callednanoparticles exist. Under the broadest definition, any particle havingone dimension measuring less than 100 nanometers (nm) (or <100×10⁻⁹ m)can qualify as a “nanoparticle” despite the fact that other dimensionsof that particle may be quite large. Even for a particle for which everydimension measures less than 100 nm, the designation of nanoparticledoes not provide information related to particle shape or itsfundamental properties, which may or may not differ from the solidmaterial from which it is made. Additionally, prior to the widespreaduse of the term nanoparticle, very small particles, some of which wouldqualify under the standard definition of nanoparticles were oftenreferred to as colloidal, which generally merely meant that they weresmall enough to exhibit Brownian motion, although whether that effectresulted from the particle size or other properties such as surfacetension was rarely specified.

Today particles that qualify as nanoparticles under one or moretraditional definition are used in multiple industrial, medical andconsumer products, and interest in their properties and methods ofproduction continues to increase.

Various processes to produce what are referred to as nanoparticles areknown in the art. For example, U.S. Pat. No. 5,585,020, issued to Beckeret al., teaches methods for creation of nanoparticles in what isconsidered a narrow size distribution (e.g., particles with an averagediameter of 73 nm with a standard deviation of 23 nm). This methodutilizes laser ablation of initial diameter target particles of lessthan 100 microns within an inert gas or vacuum system.

U.S. Pat. No. 7,374,730 teaches methods for creation of nanoparticleswithin organic liquid medium and recognizes the need for stabilizingagents such as surfactants or coating agents or other hydrocarbonmaterials capable of preventing coalescence of the nanoparticles orotherwise preventing the growth of the nanoparticles into largerentities.

U.S. Pat. No. 7,662,731 recognizes the need to prevent oxidation duringlaser sputtering/ablation, but solves this by carrying out the ablationin superfluid helium.

Picosecond ablation provides shorter pulses that reduce the time forions to form and allows a method to control size, although the poweroutput of picosecond ablation is generally significantly small, limitingquantities of material produced with relatively small ablation materialplumes.

The shape of nanoparticles is also a significant characteristic and is anecessary characteristic in defining how a nanoparticle acts, interacts,or can be acted upon. Spherical particles are desirable for theiruniform shape and repeatable characteristics.

Some nanoparticles can be grown into spheres through chemical reductionmethods (e.g., silica), while production of spherical nanoparticles fromother starting materials has traditionally been through a two stepprocess. Typically, growth of nanoparticles from non-silica startingmaterials by the similar chemical reduction methods producenon-spherical shapes such as hedrons, platelets, rods, and othernon-spherical shapes. While these methods provide good control for size,the resulting non-spherical shapes require further processing beforethey can become spherical in shape. Once the specifically shapednanoparticles have been created, laser ablation is utilized toaggressively mill them into quasi-spherical and/or spherical shapes.This process produces what could be called scrap material that isknocked off of the original non-spherical particles, and in manyinstances this scrap lacks the intranuclear bonding energy to becohesive in the carrier medium resulting in ion production. Thespherical particles are then filtered to remove the ions and unwantedscrap. Although desired spherical nanoparticles are achieved by thismethod, the process is limited in its production capacity by the size ofablation field and by the batch process of precursor materials.

Due to the relatively recent advances in nanomaterial science andresearch, as well as, the identification of unique properties related tospecific nanomaterials, standardizations for nanomaterialcharacteristics are continuing to develop. In the present application,the term nanoparticle will be used to refer to particles of any shapehaving its largest dimension less than 100 nm.

Significantly absent in the art are methods capable of producing highvolume, uniformly sized, ionically stable, nanoparticles, andparticularly spherical nanoparticles. Further absent from the art aremethods for producing nanoparticles that can be suspended within aliquid solution and more particularly within a polar liquid solutionwithout the need for surfactant or other stabilizing additives.

BRIEF SUMMARY OF THE INVENTION

The present disclosure relates to novel processes and apparatus for thepreparation of uniformly sized and stable nanoparticles. In onenon-limiting embodiment, processes and apparatus for preparing uniformlysized, ionically stable spherical nanoparticles are disclosed. In oneembodiment within the scope of the invention, when these nanoparticlesare suspended within a liquid medium no surfactants or other separationagents are required to maintain suspension and stability of theparticles. As described further below, these processes and apparatusesare particularly effective in creating uniformly sized sphericalnanoparticles from a wide range of materials. Such materials include,but are not limited to, metals, both individual elemental metals andalloys, as well as solid nonmetallic starting materials, includingindividual elements, compounds and polymers.

The first step in the creation of these uniformly sized and sphericalshaped nanoparticles is ablation of a target surface to create an ejectaevent which moves radially away from the surface of a target material.In a heavy atmosphere, i.e. a fluid medium, this ejecta event will bewhat is known as an ejecta plume which has a Knudsen boundary layerseparating the vapor within the plume (which contains the ejectamaterial) from the heavy atmosphere. In a vacuum, this ejecta eventwould be what is known as an ejecta spray. This is preferablyaccomplished by delivering a specific energy packet (typically photon orelectric energy) to the target surface, that when transferred becomesphonon energy within the target surface sufficient to break theintranuclear bonds around small clusters of atoms, and ejecting clustersaway from the target surface in an ejecta event but doing so at a ratewhich reduces residual heat within the target material that may lead toion production. It should be understood that changing the physicalproperties of the target material will affect the ablation rate of thattarget. For example, annealed metal targets such as Ag have reducedintranuclear bonding energies and thus produce particles at higher ratesfrom a constant energy delivered.

Known techniques in the art for creation of an ejecta event include theuse of laser ablation as well as electrical discharge. By knowing thebonding energy between the atoms or molecules of a given target, theabsorption band of energy for the target material, and the ionizationenergy of metallic target material, a specific energy packet can bedelivered to the target material, which is sufficient to break theintranuclear bonds, but not sufficient to create heat which results inion formation from metallic targets.

In a laser ablation system, the target material will typically haveknown wavelength absorption bands, and the delivered energy packetcontent to ablate the target material is known or can be determinedthrough known testing parameters. The profile of the laser emission canbe selected to provide the most efficient transfer of photonic energy tophonon energy within the target, such as the well known “top hat” or“gaussian” profiles, and can be further tuned to deliver photonic energypackets of a specific time duration within an overall controlled areafor an energy density that induces specific ejecta event shape, size,and density of ejecta material. Similarly in an electrical dischargesystem, bursts of electrical energy from the tip of a target anodecreates the ejecta plume near the cathode surface. Control of theelectrical energy packet content to the target surface will control theejecta plume size and density of ejecta material.

For processes to create the desired stable spherical nanoparticles bylaser ablation methods, the laser pulse length and energy of a givendelivered photonic energy packet, are typically measured in times of nogreater than nanoseconds. Energy packets from electrical dischargemethods for desire stable spherical nanoparticles are typically measuredin hundreds of volts, with pulse lengths no greater than microseconds.

As the ejecta event leaves the ablation target surface it will contain adistribution of highly energized and generally nonionic particlesranging in size from small clusters of a few atoms/molecules to largeparticles containing hundreds and even thousands of atoms/molecules. Inthe case of metallic targets the initial ejecta event will likely alsocontain a small amount of individual atoms, although minimization ofheat created in the target surface by the relatively short pulse lengthof the photonic energy packet will act to minimize such single atomproduction or ions. Not only can this mixture of particles lack uniformsize and shape for the particles, the zeta potential (ξ-potential) ofthese particles is low (<±8 mV). As a result, even with ions removed,the nonuniform forces between these particles create instability whichcan either result in particle disassociation into individual ions oragglomeration of particles together leading to precipitation out of aholding media thereby significantly reducing the quantity ofnanoparticles in a holding media.

Stability of the particles in a holding media is substantially increasedthrough uniformity of particle size and imparting ξ potential >±20 mV,and is achieved through the use of at least one electromagnetic energyfield, and more preferably a gradient electromagnetic field composed ofmultiple discrete fields of varying energy strength. This at least oneelectromagnetic field is generally parallel to the target surface andtherefore generally perpendicular to the direction of the expandingejecta event. Specifically if the ablation laser is designated as thex-axis, then the electromagnetic energy field will form y-z planes infront of the target material. Without being bound by theory, this field,or combination of multiple fields, will act on the clusters within theejecta event, where sufficient phonon energy within clusters stillexists, to induce uniformity of particle size by both causing largeparticles to split as well as causing small clusters of atoms toagglomerate with other particles thereby causing the composite particlesize distribution to narrow.

The size of the spherical nanoparticles that will remain stable in agiven environment is a function of bonding energies of atoms ormolecules within a geometric effect of the particle compared to thedisassociation energies of the particle's environment. As used hereinthe term geometric effect of the particle means where the surfaceexhibits uniform radial curvature characteristics (as opposed to planarfor large particles or point characteristics for small particles).Stability of the spherical nanoparticle, meaning that the particle isnot susceptible to significant mass loss by ions or clusters of atomsleaving the spherical nanoparticle, is believed to be achieved becausethe combined bonding energy (_(b)) resulting from the uniform geometriceffect is greater than the thermal energy of the medium (_(m)). As such,for materials such as gold (Au) and platinum (Pt) whose bonding energyis relative high, stable particles having diameters as low as 1 nm havebeen observed in polar liquids such as water compared to silver (Ag)where stability is typically not maintained until 2.5-5 nm diameters. Ithas also been observed that once the particle sizes of most materialexceeds about 35 nm the particles exhibit more planar geometric effectsleading to ionization because atoms/molecules at the surface exhibitlocalized reduced bonding behavior rather than the group geometriceffect of a stable spherical nanoparticle.

In one disclosed embodiment, the gradient electromagnetic energy fieldscould be designed to not only narrow the particle size distribution, butto also create generally spherical particles without any discretecorners or points.

Further, while the electromagnetic field in front of the target may belimited to a single energy field, in the preferred embodiment, thisfield can include at least three, and more preferably at least four,fields of varying levels of energy. These multiple fields can be createdfor example by using diffraction optics to divide a single laseremission or through the use of multiple gradient electrical fields.Size, range and distribution of the nanoparticles are influenced by thewavelength and energy of the electromagnetic field(s) sufficient to actupon the energized particles within the ejecta event as the ejecta eventexpands through the electromagnetic fields. Without being bound bytheory, it is believed the closer the electromagnetic fields can beplaced to the beginning of the ejecta event, or surface of the target,where the clusters of ejected target material are more dense andenergetic, the better the clusters are able to be acted upon by theelectromagnetic fields. Specific characteristics in the electromagneticfields of wavelength and energy density have uniforming effects on theejecta clusters resulting in narrowing size distribution and overallsize and shape of the nanoparticles.

Particles within an ejecta event leave the target highly energized andcan initially move at or near sonic velocity as they expand radiallyaway from the target surface. The velocity in an ejecta plume isaffected by the pressure of the heavy atmosphere around the target, withhigher pressures or viscosities causing a more rapid decrease in thevelocity of the particles. As such, under typical conditions, theparticles within the event continue to spread away from each other withlittle incentive or compulsion for any combination or other interactionand without any additional energy applied to the particles. However, inthe present invention as the ejecta event begins its radial expansionaway from the target surface it encounters an electromagnetic field andpreferably multiple such fields. It is believed that the wavelength andenergy of these discrete electromagnetic fields induces a uniformingeffect which results in nanoparticles of the same size and plasmonresonance to a point that the electromagnetic fields have a limitedeffect on the nanoparticles because they have reduced absorption ofenergy by the electromagnetic fields of specific frequency and maintainthe same size and plasmon resonance.

This process yields generally spherical nanoparticles of uniform sizemeaning that at least 99% of the particles will have diameters withinpreferably ±3 nm, and most preferably within <±1 nm in diameter.Additionally the particles possess high ξ potential (preferably >±30mV). This means that when suspended within any medium, including anypolar liquid such as water, these particles exert uniform forces on eachother and thereby remain suspended in solution without the need for anyadded surfactants. The lack of surfactants now allows introduction ofthese particles into applications where the presence of the surfactantswould otherwise prove problematic, such as biological systems.

Given that a single ablation step on a target will dislodge a minisculeamount of material from the target surface, the process for obtaining auseable quantity of uniformly sized nanoparticles will also include thecapacity to perform numerous ablations on the target, ideally over arelatively short time, as well as the ability to collect the particlesin a useable volume or space. When utilizing laser ablation this can beaccomplished within a reaction chamber which allows a primary ablationlaser to scan and repeatedly ablate a target surface and which furtherallows a secondary cross laser that has preferably been split withdiffraction optics or using multiple lasers (whether of different orsimilar energy densities), to provide the gradient electromagneticfields. Input and output ports on the chamber allow a heavy atmosphere,whether gas or liquid, to carry away the nanoparticles after they havepassed through the electromagnetic fields. Similarly, in a vacuumsystem, the particles of the ejecta spray will be deposited typically onthe chamber wall opposite of the target.

Additionally, in circumstances where a liquid is recirculated throughthe chamber to allow nanoparticle concentrations to build up, byminimizing the volume of solution in front of the target continueddestruction of the particles by the ongoing laser energy can beminimized.

These and other advantages and novel features of the invention will beset forth in part in the description which follows, and in part willbecome apparent to those skilled in the art upon examination of thefollowing or may be learned by practice of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a drawing of a preferred embodiment of an apparatus forperforming the process of the present invention using laser ablation;

FIG. 2 is a depiction of an ejecta plume within a heavy atmospheremoving away from a target surface and its interaction with multiplegradient electromagnetic fields;

FIG. 3 is a graph showing particle size distribution of 23 nm<±14.2 nm(as determined by dynamic light scattering) after laser ablation of a Agtarget by a laser at 1064 nm wavelength using 3.9 nanosecond pulses todeliver 500 mJ energy per pulse in a double distilled, deionized watersystem, but without the use of any gradient electromagnetic fields;

FIG. 4 is a graph showing particle size distribution of 4.6 nm<±1 nm (asdetermined by dynamic light scattering) after laser ablation of a Agtarget by a laser at 1064 nm wavelength using 3.9 nanosecond pulses todeliver 500 mJ energy per pulse in a double distilled, deionized watersystem, after the ejecta plume passed through a cross laser at 532 nmwhich laser had been split by a diffraction optic;

FIG. 5 is a transmission electron microscope image of spherical 10 nm Agparticles suspended within a water solution prepared by the presentinvention;

FIG. 6 is a drawing of a toroid that contains multiple metallicconcentric bands on its surface; and

FIG. 7 is a drawing of a preferred embodiment of an apparatus forperforming the process of the present invention using electricaldischarge.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention relates to novel processes and apparatus for thepreparation of uniformly sized and spherical-shaped nanoparticles. Asdescribed further below, these processes and apparatus are effective increating nanoparticles, specifically spherical nanoparticles from a widerange of materials, including metals, both individual elemental metalsand alloys, and nonmetallic starting materials, including polymers.

FIG. 1 shows an example of apparatus utilized within a system forproducing uniformly sized nanoparticles using laser ablation. A primarylaser 10 emits or delivers discrete energy packets of photon energy in apulsed manner 12. Typically the diameter of laser pulse emission 12exiting the primary laser 10 is expanded through beam expanding optics14 to reduce the power density to allow the emission to move throughscanning optics 16 without destroying the optic coatings. After leavingthe scanning optics 16 the laser emission 12 then typically passesthrough beam collimating optics 18 to create the desired spot size forlaser emission 12 as it enters a chamber 20 through an optic window 22and interacts with the target 24. The scanning optics 16 adjust thedirection of emission 12 slightly for each pulse to move emission 12around the surface of the target 24 and typically are either polar orx-y scanners. This precludes the laser emission 12 from repeatedlystriking the exact same location on target 24 thereby allowing foroptimal particle ablation during each pulse and providing as efficientutilization of the target as practically possible. Significantly,whether the target moves or the energy beam moves is less important thanprecluding repeated energy delivery to the same point on the target.Further, one skilled in the art will recognize that the path of laseremission 12 preferably will occur within a hermetically sealedenvironment to preserve the integrity of laser beam profile (typicallyeither a “top hat” or Gaussian profile).

The type and frequency of primary laser 10 is primarily a function ofthe target material to be ablated as well as considerations ofcommercial availability and cost of the primary laser. Typically thetarget material will typically have known wavelength absorption bands.Where no known absorption wavelength absorption bands exists for a giventarget material or where further optimization from reported values isdesired, the frequency for primary laser 10 can be experimentallydetermined by finding a suitable and strong absorption band for thespecific material to be ablated.

Further, the beam spot size and energy density will control the totalenergy delivered (E_(T)) in each energy packet or pulse for emission 12.This will be a function both of the target material's bonding energy(E_(B)) as well as the number of total atoms/molecules to be containedwithin the desired final spherical nanoparticle.

The pulse duration for laser emission 12 preferably will allow deliveryof sufficient energy within each pulse or energy packet to ablate thetarget material, while still maintaining energy content of the pulsebelow the ionization energy of the target. This maximum pulse duration(P_(D)) will be particularly significant in the instance of metallictargets and again can be determined experimentally or by dividing thetarget ionization energy (E_(I)—in joules) by the total energy deliveredfrom emission 12 (E_(T)—in joules/sec) as shown:PD=E _(I) /E _(T)

For example, typically for preparation of spherical Ag nanoparticleswith diameters less than 35 nm the pulse duration (PD) for creation of asuitable ejecta event has been found to be less than 10 nanoseconds.

The profile of the laser beam can be selected to provide the mostefficient transfer of photonic energy to phonon energy within thetarget, such as the well known “top hat” or “Gaussian” profiles, and canbe further tuned to deliver photonic energy packets of a specific timeduration within an overall controlled area for an energy density thatinduces specific ejecta event shape, size, and density of ejectamaterial.

As further shown in FIG. 1, target 24 is preferably held within thehollow reactor chamber 20 at the back end 26 of that chamber by a targetholder 28. The front end 30 of chamber 20 also contains a front optic 22which allows emission 12 to pass through on its way to target 24.Preferably, a small piezo-electrically controlled vibrator 32 is mountedinside the front end of the chamber 30 behind the front optic 22 suchthat its regular vibration precludes buildup of nanoparticles on it,thereby protecting the front optic 22. If nanoparticle buildup occurs onthe front optic 22 then propensity for damage to the optic from theincoming laser emission increases. The hollow interior of chamber 18acts to contain the ejecta event (not shown) as that plume leaves thesurface of target 24 after being impacted by each pulse from emission12.

Once a pulse from emission 12 interacts with the surface of target 24,the energy of the laser photons transfers into the lattice structure ofthe target becoming phonon energy which breaks the intranuclear bondswithin the lattice structure and releasing particles from the targetsurface. Because the bonding energies between the atoms within thelattice structure control the quantity of material that is ablated by aspecific quantity of energy delivered to the target surface, lowerbonding energies between atoms result in more rapid target materialablation. This means that processes to “soften” a target, such asannealing, have been found to significantly increase the rate of atarget's ablation. Additionally, in a preferred embodiment the target 24can be heated by a target heater 34 which will typically increase thetemperature of target 24 by approximately 10° C. over ambient conditionsto further decrease the bonding energies within the target's latticestructure.

Despite the attempt to control the energy delivered to the targetsurface to cause formation of specifically sized particles as discussedabove, the particles of an ejecta event typically will contain adistribution of uncharged, nonionic particles ranging in size from smallclusters of single digit atoms/molecules to particles of the generallydesired size as well as many even larger particles. Further, in the caseof metallic targets even with the laser energy delivered to the targetbeing less than the target's ionization energy, the initial ejecta eventwill likely also contain some ionized, individual atoms. As such,metallic targets are preferably charged as an anode and grounded throughan electrical outlet 36 so that ablated ionized atoms are drawn backinto the target and reabsorbed into the target crystalline structurethereby eliminating any free ions from the ejecta event and subsequentlyproduced nanoparticles.

In order to facilitate continuous production and removal of the ablatedparticles, chamber 20 typically contains a fluid input port 52 and fluidoutput port 54 which are connected through input 56 and output 58 tubingor piping or other similar structures to a tank 60 or other similarholding vessel or chamber that contains the desired fluid, whetherliquid or gas or other heavy atmosphere. The temperature of the fluidwithin tank 60 can be controlled through the use of a heating jacket 62or other known mechanisms and preferably will contain a mechanism formixing the fluid, whether by stirring or other mechanism. In systemsutilizing liquids, the pressure within the chamber can be controlled byadjusting the height of the output port 54. The pressure in gas systemscan be controlled by controlling the gas pressure. Similarly, in vacuumsystems, the creation and maintenance of the vacuum within the systemwill operate with commonly understood components. Tank 60 can furtherinclude a sample port 64 which could also include sensors fortemperature, pressure and/or fluid volume. Further, one skilled in theart will recognize and understand that all material surfaces within thechamber, input and output ports, tubing or piping and tanks must benonreactive, non-attractive and non-absorbent to or with the specificnanoparticles being created. For example, untreated glass and quartzwill readily absorb many types of nanoparticles, particularly metallicparticles and pose substantial problems for use as materials for thereaction chamber 20. Preferred materials include teflon, PEEK, and PET.Further, where a pump 66 is needed for a liquid system peristaltic pumpsare preferred.

Preferably flow rates will be maintained at low linear velocities abovetarget 24 to provide laminar flow through reaction chamber 20 so as toallow the particles within the ejecta event to interact with thegradient electromagnetic field(s) without interference from the fluidflow. Additionally, by minimizing the distance between target 24 and thefront 30 of reaction chamber 20, the volume of solution in front oftarget 24 through which emission 12 will pass will be minimized. Overtime the quantity of uniformly sized nanoparticles that have passedthrough the gradient electromagnetic field(s) will increase within thefluid. Since emission 12 must pass through this fluid above target 24,emission 12 has the capacity to further split the particles containedwithin the fluid. By minimizing the volume above target 24, the quantityof particles that can potentially interact with emission 12 are reducedand as such, continued destruction of the particles by the ongoing laserenergy can be minimized.

The energy packed delivered to the target 24 and the target's bondingenergies will be the primary control factors for the initial particlesize distribution within the ejecta event, which initial particle sizedistribution will largely control the size of particles ultimatelyproduced. FIG. 2 illustrates the behavior of ablated particles within anejecta plume within a heavy atmosphere as they leave the surface of atarget (i.e., not in a vacuum system which would have an ejecta spraynot an ejecta plume). Because the embodiment of FIG. 1 presumes theutilization of a heavy atmosphere, as the laser emission 12 interactswith the target 24, the ablated particles form an initial ejecta plumecontaining discrete ejecta material within the a Knudsen boundary layer38 (which boundary layer would not exist in a vacuum system). ThisKnudsen boundary layer then proceeds to expand away from the surface oftarget 24 over time as shown by the boundary layers 40-48 until theejecta plume looses all definition and the Knudsen boundary layer nolonger exists 50.

FIG. 3 provides the size distribution of particles after laser ablationof a Ag target by a Nd-YAG laser at 1064 nm wavelength using 3.9nanosecond pluses to deliver approximately 500 mJ energy per pulse, butwithout the use of any gradient electromagnetic fields. The energycontent of the laser created an average particle size of 23.15 nm with99+% of the particles being within <±14.2 nm.

Not only does the particle mixture of FIG. 3 lack uniform size and shapebut the zeta potential (ξ potential) of these particles is low (<±10mV). Consequently, the stability of these particles when suspendedwithin any liquid solution is low because of the uneven forces exertedby and on the particles. As a result, even with ions removed, the unevenforces on these particles create instability which either causesparticle disassociation into individual ions or agglomeration ofparticles together leading to precipitation out of solution therebyeliminating the nanoparticles from solution.

In order to facilitate uniformity shape and stability of the particlesand impart increased ξ potential to the particles ablated from thesurface of target 24, the system also has an electromagnetic field andpreferably multiple such fields that are substantially parallel to theface of the target 24. In the embodiment shown in FIG. 1 a set ofmultiple electromagnetic fields come from a secondary laser 68 thatemits a secondary laser beam 70. While the embodiment of FIG. 1 utilizesa laser to create the electromagnetic fields, it is understood thatmultiple other sources of electromagnetic energy including suchelectromagnetic energy sources as microwave energy can be used.

While the initial acceleration of particles leaving target 24 cantypically reach velocities at or near sonic speed, the particleaccelerations can be controlled though the use of pressure within thereaction chamber. This means that in a vacuum the near sonic speed willnot be substantially diminished as the nanoparticles move toward andultimately deposit onto the front end 30 of the chamber 20. However,where either gas or liquid medium is used to manipulate the nanoparticleflow, the pressure within reaction chamber 20 can be modified to have aneffect on the rate of accelerations in the ejecta plume, therebyproviding the particles with greater or lesser time to be affected bythe electromagnetic field(s) created by secondary laser emission 70.

In the embodiment shown in FIGS. 1 and 2, prior to secondary emission 70passing into reactor chamber, this beam passes through a holographicdiffraction grating optic 72 that produces five identifiable anddiscrete beams of differing spacial orders and different energies 74,76, 78, 80 and 82 that now can act as discrete electromagnetic fields.While five discrete beams are shown in FIG. 2, the number of such beamsmay be greater than five or less than five. The holographic diffractiongrating optic 72 will preferably allow at least 95% of the energy of thesecondary emission 70 to pass through it. Of course, rather than utilizea diffraction grating optic to create multiple discrete laser emissionsor fields from a single source laser emission, multiple individuallasers could be utilized to achieve the same effect. The frequency andintensity of these electromagnetic fields can be correlated between theabsorption bands of the target material and the plasmon resonance of theultimately desired sized spherical nanoparticle. Typically the frequencyof secondary laser emission 70 will be in the range of multiples of thefrequency of the primary laser emission 12. The frequency is preferablya frequency that is absorbed by the target material, but is absorbedless by the material once it has been ablated and forms the desiredparticle size and shape which should be a factor of the plasmonresonance of the ultimately desired sized spherical nanoparticle.

Further, a minimum energy density of the discrete electromagnetic fields74, 76, 78, 80 and 82 is necessary in order to cause the neededmanipulation of the particles caused by the ejecta event, as opposed forexample to mere observation of the ejecta event. This manipulation hasthe effect of imparting sufficient energy to the particles that willcause mis-sized particles, i.e., particles that are either larger orsmaller than desired, to either lose mass (in the case of particles thatare too large) or gain mass (in the case of particles that are toosmall) as well as cause the nanoparticles to adopt a uniform shape,typically as spheres. It is believed that this effect occurs becausemis-sized particles will more readily absorb the energy of a discreteelectromagnetic field of a specific frequency than the desired-sizedparticles. Because the desired-sized particles absorb little energy fromthe electromagnetic field, little impetus exists for these particles tochange size or shape as they move through the electromagnetic field.Conversely, because mis-sized particles will absorb the energy from theelectromagnetic field, the resulting vibrational and/or motion state ofthese particles creates an impetus for these mis-sized particles to gainor lose material so as to conform to a size and shape that is in harmonywith the electromagnetic field.

This determination of the energy density of an electromagnetic field canbegin by estimating the mass of all particles within the ejecta event(the mass of a single ejecta event can be determined by weighing thetarget before and after ablation and calculating the mass loss perejecta event). Additionally, the mass of the size and shape of theultimately desired nanoparticles is also known. When utilizing a lasermethod to create the discrete electromagnetic fields, the energy ofphotons at the given frequency is known. Therefore, a minimum quantityof photons that are necessary to cause change in a single particle to adesired size and shape can be determined experimentally. The more masswithin the ejecta event the higher required energy density of any one ofthe discrete electromagnetic fields. Further, the maximum energy densityof any one of the discrete electromagnetic fields preferably will beless than the ionization energy of the material of the desired sizednanoparticles. Once the energy densities of each of the electromagneticfields is known, then the total energy density of the secondary beam 70,which is used to create the multiple discrete electromagnetic fields,will likewise be known.

Again, as shown in FIG. 1, after passing through the diffraction gratingoptic, the now five discrete laser emissions then preferably passthrough intensifying optics 84, such as collimating lenses, that insurethat the maximum amount of energy is applied to the nanoparticles in theejecta plume. The five discrete laser emissions then pass through acylindrical lens 86 which takes the five discrete linear laser emissionsand turns them into five discrete planar laser emission that then passinto the chamber 20 through an input optic window 88 and then in frontof the target 24 and ultimately out the opposite side of chamber 20through an output optic window 90. One of skill in the art willunderstand that the optics require coatings and properties that reducelosses in a specific laser emission frequency and power for maximumefficiency and able to withstand degradation of the optics by the powerof the laser emission. Additionally, preferably, the input and outputoptics 88 and 90 respectively will also each have a piezo-electricallycontrolled vibrator 32 that can be mounted inside the chamber 30 behindboth the optics such that the regular vibration of these optics willpreclude buildup of particles, thereby protecting the both the opticsfrom particle buildup and subsequent degradation by the secondary laseremissions.

As can be seen in FIG. 2 these five discrete fields are generallyparallel to target 24 and perpendicular to primary laser emission 12such that if laser emission 12 is designated as an x-axis, then each ofthe five discrete fields form a y-z plane in front of target 24.Depending on the exact specification of the holographic diffractiongrating optic 72, the spatial orders of the discrete laser emissions ofthe five fields can be ordered. The laser emission fields closest andfarthest from target 24, i.e., fields 74 and 82 will have identicalenergy densities, as will the fields adjacent to the center, i.e.,fields 76 and 80. The center field, i.e., field 78 will be of adifferent energy density from the other two sets of fields. In oneexample, the outer fields 74 and 82 will have the lowest density, fields76 and 80 will have higher energy density and center field 78 will havethe highest energy density. In another example, the outer fields 74 and82 will have the highest density, fields 76 and 80 will have relativelylower energy density and center field 78 will have the lowest energydensity. Ideally, the first electromagnetic field 74 is at or near thetarget surface 24 such that the effect of the field on the particles isnearly instantaneous. At a minimum, it is preferred that this firstelectromagnetic field 74 act on the ejecta plume before the Knudsenboundary layer dissipates.

When properly configured, particles within the ejecta plume that havepassed through this series of electromagnetic fields are observed topossess uniformity of shape and size with >99% of the particles beingwithin less than ±1 nm has been achieved as shown in FIG. 4.Furthermore, such a process will also impart ξ potential to thenanoparticles of at least >±20 mV and preferably of ±>30 mV. FIG. 5provides an electron microscope image of 10 nm Ag particles prepared bythis method (the reason that some of the particles appear smaller thanothers relates to their position in the background or foreground of theimage).

Significantly, the present invention is not limited to the use of fiveelectromagnetic fields created from a holographic diffraction gratingoptic. For example, where only three electromagnetic fields are utilizedin place of the five fields of the above embodiment, one would expectuniformity of size less than the ±1 nm in diameter discussed above aswell as lower ξ potential. Where a single electromagnetic field is usedin place of the five fields of the above embodiment, one would expectuniformity of size and shape to be increased over a system without anyelectromagnetic field, but less than multiple electromagnetic fields.

In a further refinement of the invention, a ceramic (or othernonmetallic) toroid 92 can be mounted around target 24. As shown in FIG.6, the top side of toroid 92 can have multiple metallic concentric bands94, 96, 98 and 100 on its surface which concentric bands are eachrespectively connected to electric leads 102, 104, 106 and 108. Electricleads 102, 104, 106 and 108 are each connected to individual highvoltage power suppliers 110, 112, 114 and 116 (shown in FIG. 1) whichproduce a gradient electric field around and in front of target 24. Thisgradient electric field is used in addition to, and not in replacementof, the electromagnetic field(s) discussed above. This gradient electricfield is used to manipulate the nanoparticles' accelerations andmovement within the chamber. This gradient field is controllable bychanging voltages on the metallic concentric bands 94, 96, 98 and 100.For example, in a vacuum process, the nanoparticles' movement can becontrolled by the gradient electric field.

Because target 24 is depicted in FIG. 1 as having a significant lengthfor this specific embodiment some additional components will ideally beincluded to maintain the target surface at the desired distance from theprimary laser 10 so as both to maintain the focal point for the primarylaser emission 12 as well as the spacial relationship of theelectromagnetic fields 74, 76, 78, 80 and 82 with the target surface soas to retain consistent effects of these fields on the particles withinthe ejecta plume. In the embodiment shown in FIG. 1 this is accomplishedthrough a screw mechanism 118 which moves a pinion bar 120 that can movetarget 24 forward as the target surface is ablated by laser 12, althoughone of skill in the art will recognize that other mechanisms can be usedas well. A detector 122 can be used to monitor the position of the faceof target 24 by multiple known methods including by monitoring the firstof the electromagnetic fields 74 for a slight interruption by the targetface. Conversely, rather than moving target 24 the focal point for laser12 and the position of the electromagnetic fields 74, 76, 78, 80 and 82can be changed as the target face moves due to the loss of material fromrepeated ablations. Similarly, rather than using large targets, smalland thin targets can be utilized or the same effect can be achieved ifthe targets are routinely changed. In yet a further refinement of theinvention, multiple targets can be loaded into a target containmentvessel 124 which can act in conjunction with the screw mechanism 118 andpinion bar 120 to allow for multiple targets to be ablated without theneed to manually insert a new target into the chamber 20.

As the nanoparticles exit the gradient electromagnetic field(s) theprocess has now produced nanoparticles with high ξ potential(preferably >±30 mV). This means that these nanoparticles, whensuspended within any liquid, including any polar liquid such as water,exert uniform forces on each other and thereby remain suspended insolution without the need for any added surfactants. The lack ofsurfactants now allows introduction of these nanoparticles intoapplications where the presence of the surfactants would otherwise proveproblematic, such as biological systems.

When utilizing a liquid as the carrier for the nanoparticles, anyorganic, non-polar compound can be used as well as polar solutionsincluding alcohols and water. Preferably the chosen liquid will be freefrom ions and particulate matter to prevent unwanted agglomeration ofthe particles to impurities within the liquid. When using water,multiple methods exist to remove ionic and particulate matter includingdistillation and even multiple distillations, reverse osmosis,deionization techniques and ultrafiltration.

FIG. 7 provides an example of another embodiment of the presentinvention wherein the ejecta plume is created by an electric dischargeprocess instead of by laser ablation. Since the electric discharge willproduce ablation in a vacuum system, only an ejecta plume is created insuch a system. As will be readily understood by one skilled in the art,many of the same principles that apply to the laser ablation processwill apply to the process to ablate material using an electric dischargemethod. For example, rather than a primary laser beam impacting atarget, the electric discharge process utilizes a target anode 126 tocreate the ejecta plume near the surface 128 of a cathode material 130.In the embodiment shown in FIG. 7, this is accomplished by placing thecathode material 130 inside a holder 134 which contains a permanentmagnet 132. The holder 134 is held within a tube 136 that is part of achamber 140. An electromagnet 141 extends around the tube 136 and whenenergized creates a magnetic field that drives the holder 134 upwardstoward the tip 138 of the target anode wire 126. The electric potentialdifference between the anode wire 126 and the cathode material 130 issufficient to break down the resistance of the heavy atmosphere betweenthe surface 128 of the cathode material and the tip 138 of the anodewire which discharge creates an ejecta plume of material from the anodethat moves toward the surface of the cathode material and which theneffectively bounces off of the curved cathode surface 128 and then movesthrough the electromagnetic field(s) 153. The upward movement of theholder 134 can be limited either by a piston-type control from thebottom of the holder or even by the physical interaction of the cathodesurface 128 with the tip 138 of the anode wire 126. As the anode wireloses mass through the ablation of its tip, its length can be maintainedwith a wire feeding mechanism 142.

With the ejecta plume formed in the same location on each upward pulseof the cathode material 130 and moving within the main cavity of chamber140, an electromagnetic field or preferably gradient electromagneticfields can then be introduced into the main cavity of chamber 140through an optic window 144 at one end of chamber 140 while exitingthrough a second option window 148 at the other end. As with the opticsshown in FIG. 1, the input and output optics 144 and 148 can alsoinclude piezo-electrically controlled vibrators 150 to help preventparticle buildup on the optics. The frequency and strength of thatelectromagnetic field or fields 153, whether created from a secondarylaser 152 (or set of lasers) or other sources will be determined by thesame parameters as those described above for the gradientelectromagnetic fields in FIGS. 1 and 2.

Fluid flow can be introduced into the chamber 140 through an input port146 and exit through an output port 154 which fluid can be used tocollect the nanoparticles after they have passed through theelectromagnetic field(s). Additionally, one of skill in the art canreadily understand how this single arrangement of an anode wire andcathode material can be replicated, preferably in a linear manner, toutilize the same electromagnetic field or multiple gradientelectromagnetic fields for multiple anode-cathode units in order toincrease production of nanoparticles.

Consistent with the creation of an ejecta plume using laser ablation,the strength and duration of the electrical pulse from the tip 138 ofthe anode wire 126 will determine the total energy delivered (E_(T)) perpulse and will be a function both of the target material's bondingenergy (E_(B)), the ionization energy (E_(I)) as well as the number oftotal atoms/molecules to be contained within the desired final sphericalnanoparticle.

Even with the attempt to control particle size through precise energydelivery to the target surface, as with the ejecta plume created bylaser ablation, the plume will contain a distribution of uncharged,nonionic particles ranging in size from small clusters of single digitatoms/molecules to particles of the generally desired size as well asmany even larger particles. Further, because the electrical dischargemethod will almost always utilize metallic targets (because they act asthe anode of the electric circuit), even though the energy delivered tothe target will be less than the target's ionization energy, the initialejecta plume will likely also contain some ionized, individual atoms.However, because the target wire itself is already an anode, the ionizedatoms will readily be pulled back to the anode target and reabsorbedinto the crystalline matrix of the material.

Similarly, control of the velocity of the ejecta plume can also beaccomplished through use of the fluid pressure within the reactionchamber in the same manner as discussed above with the laser ablationmethod.

The following examples are given to illustrate various embodimentswithin, and aspects of, the scope of the present invention. These aregiven by way of example only, and it is understood that the followingexamples are not comprehensive or exhaustive of the many types ofembodiments of the present invention that can be prepared in accordancewith the present invention.

EXAMPLE 1

A Ag target was held within a chamber through which flowed tripledistilled deionized water. The Ag target was ablated using a primarylaser with a 1064 nm wavelength at 80 mJ with a 1 mm focal spot size andwith 9 nanosecond pulse lengths. The secondary laser was a continuous532 nm laser with 0.5 W power going into the diffraction grating whichcreated three distinct electromagnetic fields in front of the Ag target.The process created 10 nm diameter Ag spheres with 99+% of those sphereswithin ±1 nm diameter.

EXAMPLE 2

A Ag target was held within a chamber through which flowed tripledistilled deionized water. The Ag target was ablated using a primarylaser with a 1064 nm wavelength at 620 mJ with a 6 mm focal spot sizeand with 3.7 nanosecond pulse lengths. The secondary laser was acontinuous 532 nm laser with 0.5 W power going into the diffractiongrating which created five distinct electromagnetic fields in front ofthe Ag target. The process created 14 nm diameter Ag spheres with 99+%of those spheres within ±1 nm diameter.

EXAMPLE 3

A Ag anode wire target was ablated through a high voltage (800 V)between the target anode and a grounded Ag cathode both were submergedinto a chamber through which flowed triple distilled deionized water.The secondary laser was a continuous 1064 nm laser with 5 W power thatwas not divided with any diffraction grating optics. The process created10 nm diameter Ag spheres with 99+% of those spheres within ±1 nmdiameter.

EXAMPLE 4

A Cu target was held within a chamber through which flowed tripledistilled deionized water. The Cu target was ablated using a primarylaser with a 1064 nm wavelength at 80 mJ with a 1 mm focal spot size andwith 9 nanosecond pulse lengths. The secondary laser was a continuous264 nm laser with 0.25 W power going into the diffraction grating whichcreated three distinct electromagnetic fields in front of the Cu target.The process created 8 nm diameter Cu spheres with 99+% of those sphereswithin ±1 nm diameter.

The present invention may be embodied in other specific forms withoutdeparting from its spirit or essential characteristics. The describedembodiments are to be considered in all respects only as illustrativeand not restrictive. The scope of the invention is, therefore, indicatedby the appended claims rather than by the foregoing description. Allchanges which come within the meaning and range of equivalency of theclaims are to be embraced within their scope.

What is claimed and desired to be secured by United States LettersPatent is:
 1. A stable nanoparticle composition comprising: a polarliquid selected from the group consisting of deionized water, alcohol,and a solution thereof; and metal nanoparticles that are nonionic andhave a mean diameter less than 35 nm and a ξ potential greater than ±20mV so as to remain dispersed in the polar liquid without a surfactant,wherein at least 99% of the metal nanoparticles have diameters within ±3nm of the mean diameter, wherein the metal nanoparticles are selectedfrom the group consisting of silver, gold, copper, platinum, and analloy of silver and gold, wherein the nanoparticle composition issubstantially free of metal ions from the metal nanoparticles.
 2. Astable nanoparticle composition as in claim 1, wherein the polar liquidconsists of deionized water or alcohol.
 3. A stable nanoparticlecomposition as in claim 1, wherein the polar liquid consists ofdistilled deionized water that is substantially free of particles otherthan the metal nanoparticles.
 4. A stable nanoparticle composition as inclaim 1, wherein the metal nanoparticles consist of gold, silver or analloy of gold and silver.
 5. A stable nanoparticle composition as inclaim 1, wherein the metal nanoparticles consist of silver.
 6. A stablenanoparticle composition as in claim 1, wherein the metal nanoparticleshave a diameter of 10 nm or less.
 7. A stable nanoparticle compositionas in claim 1, wherein the metal nanoparticles have a diameter of 5 nmor less.
 8. A stable nanoparticle composition as in claim 1, wherein themetal nanoparticles are substantially spherical.
 9. A stablenanoparticle composition as in claim 1, wherein the metal nanoparticleshave a ξ potential greater than ±30 mV.
 10. A stable nanoparticlecomposition as in claim 1, at least 99% of the metal nanoparticles havediameters within ±1 nm of the mean diameter.
 11. A nanoparticlecomposition comprising: a polar liquid selected from the groupconsisting of deionized water, alcohol, and a solution thereof; andsubstantially spherical metal nanoparticles that are nonionic and have amean diameter less than 35 nm and a ξ potential greater than ±30 mV soas to remain dispersed in the polar liquid without a surfactant, whereinat least 99% of the metal nanoparticles have diameters within ±3 nm ofthe mean diameter, wherein the metal nanoparticles are selected from thegroup consisting of silver, gold, and an alloy of silver and gold,wherein the nanoparticle composition is substantially free of metal ionsfrom the metal nanoparticles.
 12. A nanoparticle composition as in claim11, wherein at least 99% of the metal nanoparticles have diameterswithin ±1 nm of the mean diameter.
 13. A nanoparticle composition as inclaim 12, wherein the metal nanoparticles have a diameter of 10 nm orless.
 14. A nanoparticle composition as in claim 11, wherein the polarliquid is a polar solution that consists of deionized water.
 15. Ananoparticle composition comprising: a polar liquid solution comprisingdeionized water; and substantially spherical metal nanoparticles ofsubstantially uniform size, wherein the metal nanoparticles consist ofsilver metal, gold metal, or an alloy of silver and gold metal and havea mean diameter less than 35 nm and a ξ potential greater than ±20 mV soas to remain dispersed in the polar liquid solution without asurfactant, wherein at least 99% of the metal nanoparticles havediameters within ±3 nm of the mean diameter, and wherein thenanoparticle composition is substantially free of metal ions from themetal nanoparticles.
 16. A nanoparticle composition as in claim 15,wherein at least 99% of the metal nanoparticles have diameters within ±1nm of the mean diameter.
 17. A nanoparticle composition as in claim 15,wherein the metal nanoparticles have a diameter of 8 nm or less.
 18. Ananoparticle composition as in claim 15, wherein the metal nanoparticleshave a ξ potential greater than ±30 mV.